Low-pressure mercury vapor discharge lamp

Abstract

A low-pressure mercury vapor discharge lamp having a very satisfactory color rendition, (R(a,8)≧85), a color temperature of 2300-3300 K and a color point on or near the Planckian curve. The lamp is provided with a luminescent layer comprising: a. a luminescent alkaline earth metal halophosphate activated by Sb 3+ and Mn 2+ having a color temperature of 2900-5000 K; b. a luminescent material activated by Eu 2+ with an emission maximum van 470-500 nm and a half-value width of at most 90 nm, and c. a luminescent rare earth metal metaborate activated by Ce 3+ and Mn 2+ , having a fundamental lattice Ln(Mg,Zn,Cd) B 5 O 10 , in which Ln represents the elements Y, La and/or Gd, which borate has red Mn 2+ emission. Further, the lamp is provided with means for absorbing blue radiation having wavelengths below 480 nm. Preferably, the luminescent layer further contains: d. a luminescent material activated by Tb 3+ which exhibits green Tb 3+ emission. Besides a very satisfactory color rendition at a low color temperature, these lamps have a high luminous flux and a high maintenance of the luminous flux during their life.

We claim: 
     
       1. A low-pressure mercury vapor discharge lamp having a color rendering index R(a,8) of at least 85, a luminous flux of at least 55 l/mW a color temperature of the emitted white light in the range of 2300 to 3300 K. and a color point on or not greater than 20 Minimum Perceptible Color Difference from the point on the Planckian curve of the same color temperature and provided with a gas-tight radiation-transparent envelope containing mercury, a rare gas, a pair of spaced elctrodes for providing a discharge, a luminescent layer comprising: a. at least one luminescent alkaline earth metal halophosphate activated by trivalent antimony and bivalent manganese and having a color temperature of the emitted radiation of 2900 to 5000 K.;   b. at least one luminescent material activated by bivalent europium with an emission maximum in the range of 470 to 500 nm and a half-value width of the emission band of at most 90 nm, and   c. a luminescent rare earth metal metaborate activated by trivalent cerium and bivalent manganese, having a monoclinic crystal structure, whose fundamental lattice corresponds to the formula Ln(Mg,Zn,Cd)B 5  O 10 , in which Ln is at least one of the elements yttrium, lanthanum and gadolinium and in which up to 20 mol.% of the B can be replaced by Al and/or Ga, which metaborate exhibits red Mn 2+  emission, and   means for absorbing at least in part blue radiation having wavelengths below 480 nm.   
     
     
       2. A lamp as claimed in claim 1, wherein the luminescent layer further contains a luminescent material activated by trivalent terbium (material d), which exhibits green Tb 3+  emission. 
     
     
       3. A lamp as claimed in claim 2, wherein that the luminescent metaborate c is further activated by trivalent terbium, the metaborate c being at the same time the material d, and corresponds to the formula   (Y,La,Gd).sub.1-f-g Ce.sub.f Tb.sub.g (Mg,Zn,Cd).sub.1-h Mn.sub.h B.sub.5 O.sub.10,     in which     0.01≦f≦1--g       0.01≦g≦0.75       0.01≦h≦0.30 and in which up to 20 mol. % of the B is replaceable by Al and/or Ga.     
     
     
       4. A low-pressure mercury vapor discharge lamp as claimed in claim 3 wherein the color rendering index R(a,8) is at least about 90. 
     
     
       5. A lamp as claimed in claim 4, wherein the means for absorbing blue radiation is the radiation-transparent envelope of the lamp. 
     
     
       6. A lamp as claimed in claim 4, wherein the means for absorbing blue radiation is a yellow pigment. 
     
     
       7. A lamp as claimed in claim 6, wherein the pigment is mixed with the luminescent materials of the luminescent layer. 
     
     
       8. A lamp as claimed in claim 6, wherein the pigment is provided on the inner side of the lamp envelope as an absorption layer, on which the luminescent layer is disposed on the side facing the discharge. 
     
     
       9. A lamp as claimed in claim 1, wherein the means for absorbing blue radiation is a luminescent aluminate activated by trivalent cerium having a garnet crystal structure according to the formula M 3-j  Ce j  Al 5-k-p  Ca k  Sc p  O 12 , in which M is at least one of the elements yttrium, gadolinium, lanthanum and lutetium and in which   0.01≦j≦0.15       0≦k≦3,       0≦p≦1.     
     
     
       10. A lamp as claimed in claim 9, wherein M is yttrium and k=p=0. 
     
     
       11. A lamp as claimed in claim 9, wherein the garnet activated by Ce 3+  is mixed with the remaining luminescent materials of the luminescent layer. 
     
     
       12. A lamp as claimed in claim 9, wherein the garnet activated by Ce 3+  is applied on the inner side of the lamp envelope as an absorption layer on which the luminescent layer is disposed on the side facing the discharge. 
     
     
       13. A lamp as claimed in claim 4, wherein the material b is a luminescent aluminate activated by bivalent europium corresponding to the formula   Sr.sub.1-q-r Ca.sub.q Eu.sub.r Al.sub.s O.sub.11/2s+1,     in which     0≦q≦0.25,       0.001≦r≦0.10 and       2≦s≦5,     which aluminate has its emission maximum at 485-495 nm and has a half-value width of 55-75 nm.   
     
     
       14. A lamp as claimed in of claim 4, wherein the material b is a luminescent aluminate activated by bivalent europium corresponding to the formula   Ba.sub.1-t-r Sr.sub.t Eu.sub.r Al.sub.s O.sub.11/2s+1     in which     0≦t≦0.25,       0.005≦r≦0.25 and       5≦s≦10,     which aluminate has its emission maximum at 475-485 nm and has a half-value width of 70-90 nm.   
     
     
       15. A lamp as claimed in any one of claims 4 to 11, wherein the material b is a luminescent borate phosphate activated by bivalent europium corresponding to the formula m(Sr 1-v-w-z  Ba v  Ca w  Eu z  O).(1-n)P 2  O 5 . nB 2  O 3 , in which   0≦v≦0.5       0≦w≦0.2       0.001≦z≦0.15       1.75≦m≦2.30       0.05≦n≦0.23,     which borate-phosphate has its emission maximum at 470-485 nm and has a half-value width of 80-90 nm.